1. Field of the Invention
The present invention relates to an earth excavator. More particularly, the invention relates to an earth excavator including means for stabilizing performance by compensating for variations in temperature.
2. Description of the Prior Art
In large earth excavators of the dragline or shovel type such as are used in mining operations, or in large earth-boring drills, a number of electric motors are included to provide motive power. One or a plurality of relatively large generators are also provided to supply electric current to the armatures of the various motors. Additionally, a relatively small DC generator, known as an exciter or exciting generator, is normally mounted on the equipment to supply constant field or excitation current to the field windings of the various motors. All of the generators are typically driven by a single prime mover which may be an AC electric motor supplied by commercial AC power lines, an internal combustion engine, or any other suitable power source. Each of the generators, including the exciting generator, may be combined, in a well-known manner, with an AC electric motor to form a motor-generator set.
An earth excavator is typically operated under widely varying ambient temperature conditions, which may range from -40.degree. C. to +40.degree. C. The performance of the motors and thus of the excavator is adversely affected by these extreme variations in temperature, as will now be explained. The electrical resistance of the various motor field windings varies with winding temperature. Decreases and increases in winding resistance with variations in temperature result in corresponding increases and decreases in motor field winding current and corresponding increases and decreases in power output and torque available from the various motors and thus variations in performance of the excavator. Absent appropriate temperature compensation for the drive motors, an operator of the excavator must learn to compensate for these variations in performance of the excavator. In general, he will be aware that at relatively low temperatures the motors develop their greatest torque. He must also know that the torque developed by some of the drive motors at low temperatures may exceed the strength of various structural members of the excavator in which the motors are used and could cause breakage thereof unless the operator promptly recognizes and compensates for such excessive torque.
Prior art attempts to deal with the problem of variations in performance of an excavator with variations in temperature include the provision of one or more auxiliary field windings in its exciting generator and the incorporation of one or more large negative temperature coefficient thermistors electrically connected in series with the auxiliary field windings and physically located in proximity thereto in order to sense the temperature thereof. With such an arrangement, as the sensed temperature of the exciting generator field windings decreases, an increase in resistance of the associated thermistor causes a decrease in field current through the exciting generator field windings. This causes a corresponding decrease in output voltage from the exciting generator armature and thus a decrease in voltage supplied to the motor field windings.
One design difficulty presented by such thermistor circuits involves the selection of a suitable location for the high wattage thermistors that are used. Since it is desired to compensate for changes in resistance of the motor field windings, caused by changes in motor field winding temperature, a theoretically optimum location for the thermistors would appear to be near a motor field winding. However, in practice it has been found, when other factors are considered, that location of the thermistors near the exciting generator field winding is a better compromise. For one thing, it is usually much easier to locate the thermistors physically near the field winding they are electrically connected in series with. For another, it is primarily changes in performance with ambient temperature that are to be compensated for and the temperature of the exciting generator and the temperature of the motors are likely to be affected similarly by changes in ambient temperature. Thus, the exciting generator field winding and the motor field winding rise in temperature during normal operation of the equipment at much the same rate. Additionally, if the thermistors were to be placed near a motor field winding, a choice would have to be made of which motor to use, since there may be several motors in the equipment, and this choice may itself require a compromise.
The use of large thermistors in this manner, while providing some degree of temperature compensation, is not completely satisfactory for a number of reasons. Even the highest-wattage thermistor available is often not really large enough and may have excessive internal heating when utilized as indicated. Adjustable high-wattage resistors are required to be connected both in series and parallel with the thermistors in order to provide adequate adjustment, and these resistors require elaborate mountings and protection means to shield personnel from the heat developed and from possible electric shock. Moreover, it is very difficult to adjust an excavator including large thermistors to achieve a satisfactory exciting generator output voltage versus field winding temperature characteristic.
A further disadvantage of such prior art excavators including large temperature compensating thermistors is that, while a degree of temperature compensation is achieved, there is no actual regulation of the output voltage from the exciting generator. Changes in electrical loading of the exciting generator or changes in rotational velocity at which the exciting generator is mechanically driven result in adverse variations in exciting generator output voltage.
Besides the foregoing operating difficulties, the manufacturing expense associated with such prior art temperature compensating arrangements is undesirably high. In a typical large excavator as many as eight thermistors and associated high-wattage resistors may be required to approach adequate compensation. Such thermistors now cost in excess of $40.00 each.